An induction generator (IG), as is commonly known in the art, is a type of electrical generator that is mechanically and electrically similar to a polyphase induction motor (IM). An IG produces electrical power when its shaft is rotated faster than the synchronous frequency of the equivalent IM. IGs are often used in energy conversion systems or wind turbines and some micro hydro installations due to their ability to produce useful power at marginally varying rotor speeds. IGs are also generally mechanically and electrically simpler than other generator types. They are also more rugged and require no brushes or commutators.
IGs are, however, not self-exciting, meaning they require an external electrical supply to produce a rotating magnetic flux. The external supply can be supplied from an electrical grid or from the generator itself, once it starts producing power. The rotating magnetic flux from the stator induces currents in the rotor, which in turn also produces a magnetic field. If the rotor turns slower than the rate of the rotating flux, the machine acts like an induction motor. If the rotor is turned faster, it acts like a generator, producing power at the synchronous frequency.
In most IGs the magnetising flux is established by a capacitor bank connected to the machine in case of stand-alone systems. In the case of grid connected systems it draws magnetising current from the grid. IGs are suitable for wind energy conversion systems as speed is always a variable factor in these applications.
The concept of having an internally excited permanent magnet induction generator (PMIG) is known. These generators function on the principle of having an additional, freely rotating permanent magnet (PM) rotor in combination with the normal induction rotor, generally positioned between the induction rotor and the stator. The PM rotor provides the flux within the machine, thus alleviating the need for a magnetizing current which, in turn, results in an improved power factor for the machine as a whole.
Most commercially available wind energy conversion systems currently utilise a combination of complex gearboxes and high speed IMs. These systems are generally directly connected to an electricity grid, which is made possible by the IM being capable of slipping, thus allowing for a soft grid connection.
A popular alternative in wind energy conversion system layout and design is the low speed permanent magnet synchronous machine (PMSM). The layout of a typical drive train is shown in FIG. 1. The drive train can represent either an induction machine or a synchronous machine. If, for example, the gearbox is omitted the drive train can represent a PMSM and if the converter is omitted it can represent an induction machine. The PMSM may also rely on a full frequency electronic power converter to change the voltage level and the frequency of the generated power, so as to allow it to connect directly into the electricity grid. In what follows, the term power converter will refer to a full frequency electronic power converter. A system, such as the one shown, which does not utilise a gearbox is known as a direct drive system.
More variations like the doubly fed induction generator (DFIG) which is used regularly in the wind turbine industry, combinations of PMSMs and gearboxes, or IMs and converters are used on a limited scale (i.e. mostly utility scale) in some markets. To the applicant's knowledge, wind turbine systems currently in use generally consist of the electrical machine which is operated in conjunction with a gearbox, a power converter or both.
Because most conventional wind turbines generally operate at low rotational speeds, gearboxes are needed in order to use them with high speed IMs. Without a converter of sorts, IMs can only operate as high speed devices due to the large increase in the magnetizing current for low speed, directly grid connected induction machines. PMSMs on the other hand can operate efficiently at low rotational speeds but cannot be directly connected to an electricity grid in a wind energy conversion system.
Gearboxes and power converters used in conventional wind energy conversion systems are mechanically complex, expensive, maintenance intensive pieces of equipment, which increase the overall cost of the overall system. Gearboxes also contribute substantially to the overall system mass and losses due to, for example, heat and noise. Power converters, on the other hand, are complex and expensive, electrically sensitive systems.
The layout of a typical PMIG is shown in FIG. 2. The PMIG consists of an ordinary stator, an induction type cage rotor and an additional, free rotating PM rotor between the stator and rotor of an induction machine or in the inside of the rotor (or outside of the stator), as is more clearly shown in FIG. 3. When used in a wind turbine, the mechanical shaft power which is supplied by the wind turbine rotor to the electrical machine is transmitted to the cage induction rotor, while the PM rotor rotates freely and independently on its own shaft. The PM rotor supplies the magnetic flux within the electrical machine and induces a voltage in the stator winding as shown in the equivalent electrical circuit layout of FIG. 4. This, in principle, reduces the magnetizing current and improves the power factor of the machine. These generators typically make use of standard stator and cage rotor windings. It has, however, been found that there is a cogging (torque) effect between the PM rotor and the stator or rotor. Cogging causes the PM rotor to lock with respect to the stator core or cage-rotor core, which causes instability at low slip speeds.
The advantages of PMIGs for wind, as well as other, generator applications are very attractive as it avoids the use of gearboxes and the use of power converters for grid-connection. The device is therefore a direct-drive direct-grid wind energy converter which is a very attractive concept. But in spite of these obvious advantages, to the applicant's knowledge, no PMIG wind generator has as yet been installed or tested. The main reasons for this appear to be the difficult construction of the machines.